Police spokesman Maj. Gen. Abdul-Karim Khalaf said the Americans were being held at an Iraqi police station inside the Green Zone "in connection with a joint U.S.-Iraqi investigation" into Kitterman's death but gave no further details.
"Until now, the detained persons are suspects and no formal charges have been filed against them," he told The Associated Press.
U.S Embassy spokesman James Fennell confirmed that five Americans were in Iraqi custody but said no formal charges have been filed so he couldn't provide further details about the detention.
A U.S. official familiar with the case said the five were being investigated for allegations other than murder. He spoke on condition of anonymity because the investigation was continuing.
The five were believed to be the first Americans taken into Iraqi custody since the U.S.-Iraqi security agreement went into effect this year. The agreement removed immunity from Iraqi law enjoyed by private U.S. contractors since the collapse of Saddam Hussein's regime in 2003.
Embassy officials have visited the men to make sure they're being given their rights in accordance with Iraqi law, Fennell said, adding "the men appeared well."
He said FBI agents were present during a search of the men's quarters at the request of Iraqi authorities who are handling the investigation.
U.S. and Iraqi authorities declined to identify the contractors.
However, an official of Corporate Training Unlimited, a Fayetteville, N.C.-based security company, said the five included Donald Feeney Jr., 55, who founded the company in 1986, his son Donald Feeney III, 31, and three other employees.
"I think everybody is devastated by the loss, including the Feeneys. And they're cooperating fully with the investigation," company spokeswoman Sarah Smith said. "They've not been charged with anything. And we suspect that they won't be charged with anything."
She said the CTU contractors knew Kitterman as "simply comrades living in the Green Zone."
"They ate meals together and just knew each other, I guess, being around one another. I think there was a mutual respect for one another and I know for a fact that the Feeneys are really devastated by his loss. The day he was murdered, I got a call saying how upset everyone was," she said.
CTU trains corporate officials on how to avoid terrorists while they are overseas. The company, which has operated in Iraq since 2003, also has gained attention for rescuing American children taken to foreign countries in custody disputes.
Kitterman had also been working in Iraq since 2003 and founded a small construction company that operates from the Green Zone.
Although Americans and others have been killed in rocket or mortar attacks in the Green Zone, Kitterman was believed to be the first American ever slain in a criminal act since the protected area was established after the city fell to U.S. forces in April 2003.
Iraq assumed control of the Green Zone on Jan. 1 under the security agreement, taking primary responsibility from the Americans for searching vehicles and checking identity papers as entry checkpoints.
The Iraqis have begun removing some of the protective blast falls around the Green Zone — part of a campaign to restore a sense of normalcy as violence in the city has waned.
Violence, however, continues.
A rocket or mortar slammed into the Green Zone Sunday morning but no casualties were reported, according to the U.S. military.
The attack came just over two weeks after an American was killed when a rocket struck the sprawling area that houses the U.S. Embassy and much of the Iraqi government.